The present invention relates to a solid matrix for use with surface-enhanced Raman spectroscopy, and a method of making such a matrix.
Spontaneous Raman scattering from chemical compounds is an inherently weak effect; only a very small proportion of all photons on a normal non-absorbing sample will be Raman scattered. Increased scattering probabilities may be induced by use of the xe2x80x9cresonance Raman effectxe2x80x9d where the wavelength of the light source is chosen to fall on, or near, an electronic absorbtion band of the sample. However, resonance Raman effects can only be used for samples in which the compound of interest has electronic absorbtion band at one of the excitation wavelengths which are available to the experimenter.
A second method of increasing Raman scattering signals is to use xe2x80x9csurface-enhancementxe2x80x9d in so-called xe2x80x9csurface-enhanced Raman spectroscopy (SERS)xe2x80x9d. Under conditions where surface and resonance enhancement both operate, the technique is termed xe2x80x9csurface-enhanced resonance Raman spectroscopy (SERRS)xe2x80x9d. Surface-enhancement of Raman signals is observed when the species of interest is absorbed on, or near to, a microscopically-rough metal surface. Not all metals give rise to the effect; the two most commonly-used metals are silver and gold. A very broad range of methods have been used to treat these metals to give surfaces which enhance Raman signals of chemical compounds (ie which are SER(R)S-active). The most widely used are those which involve electrochemical or chemical roughening of metal surfaces, deposition of the metal onto substrate (for example by preparation of metal island films), and preparation of colloidal suspensions of the metals, normally in aqueous solution, hereinafter referred to as sols.
SER(R)S can be an extremely sensitive technique (reports of single molecule detection have been published recently) and has good discrimination because it yields vibrational spectra of compounds which are characteristic of each particular compound. The combination of sensitivity and discrimination makes SER(R)S an obvious technique for the analysis of a very broad range of chemical substances. However, although the potential of the technique is clear, there has been very little exploitation of the technique for routine analytical tasks. The main obstacle to routine analysis is that of signal reproducibility. There are two main sources of this irreproducibility:
1 Irreproducibility in presentation of the sample to the Raman excitation/collection optical system. This is a purely mechanical problem and will not be considered further here.
2 Irreproducibility in the SER(R)S-active surfaces. This is a significant problem. If roughened electrodes are used, the roughening procedure must be exactly replicated between measurements and, even if this is possible, contamination of the surface by highly-scattering compounds is difficult to eliminate. The contamination problem can be removed if a completely fresh surface is used for each measurement. The easiest way to ensure fresh surfaces for measurement is to use small aliquots of colloidal solutions (which are inexpensive to prepare) and then discard them. However, it is widely recognised that preparation of colloidal solutions with identical surface-enhancing properties is extraordinary difficult. Moreover, the solutions themselves are inherently unstable and may decay over time or due to the presence of trace amounts of chemical impurities.
An object of the present invention is to combine the low production cost of colloidal suspensions with the ability to produce large numbers of identical and stable SER(R)S active materials. Moreover, these materials should be presented in forms which are convenient to handle and manipulate but sufficiently inexpensive that they can be used once and then discarded.
According to one aspect of the present invention, there is provided a method of forming a solid matrix for use with surface-enhanced Raman spectroscopy, comprising the steps of:
admixing a colloidal metal solution with a polymeric support medium to form a suspension; and drying the suspension to form the matrix.
The polymeric support medium surrounds the particles in the colloidal metal solution (sol), in effect giving a polymer/sol suspension. When in this form, the sol particles are resistant to aggregation and precipitation, but are still accessible to any solvent-borne analyte. Indeed, it has been found that such suspensions are stable, in that they show no discernable spectral changes over several months.
The term xe2x80x9csuspensionxe2x80x9d as used herein means any solid and/or liquid form of the combination of the components, including gels and emulsions.
Complete drying leaves a mechanically-hard, transparent or translucent film. In the dry films, the metal particles are not only prevented from aggregating, but they are also protected from environmental damage. The films show no discernable change after several months in storage. An inert matrix has been formed from the metal particles and the dry polymeric support medium, although no active bonding has been formed between these substances. Such a matrix can then be used as a support surface for analysis by SER(R)S.
To (re)activate the solid matrix, it can then be treated with an appropriate solvent, or more commonly a solution of analyte, at which point the matrix swells due to solvent ingress and the particles contained within are then free again to interact with the chemical substance to be analysed. Most importantly, the matrix regains its ability to produce surface-enhancement after they have been re-solvated.
Any suitable polymeric support medium may be used in the present invention, which is able to form a suspension with sols formed for use in SER(R)S, and does not reduce their surface enhancement properties. Sols are generally considered to be xe2x80x9cunstablexe2x80x9d, due to the very small particles therein, which are therefore very sensitive to any change, especially any change in environment. The polymeric support medium provides support for the sol metal particles, both in any liquid form, or in any solid form.
Suitable polymeric support media include any known absorbents, or hydrophillic swelling polymers, such as those with carboxylic side chains, including polymers such as polycarbophil, copolymers such as hydroxyethylmethacrylate with methacrylic acid (xe2x80x9cHEMAxe2x80x9d), polyvinylmethyl maleic anhydride ester, and cellulose-based substances such as hydroxyethylcellulose.
The polymeric support medium may be solid or liquid.
Where the polymeric support medium is wholly or substantially a liquid or gel, etc, ie the suspension formed is at least not a solid, the suspension is applied or deposited on a surface, e.g. spread across a support, and then dried in air or under a vacuum, etc. During the drying, the polymer/sol suspension shrinks as the polymeric support medium returns to an anhydrous state.
The surfaces on which the polymer/sol suspension can be applied to include any form, design or shape. One common form is a flat plate, generally of clear glass. Alternatively, the suspension could be applied to multiple plates or wells, eg. a (standard) microwell plate. The suspension could also be added to eg the inside of capillary tubes or pipettes, which tubes or pipettes can be used to draw up very small samples of analyte directly.
Preferably, any liquid suspension has at least some viscosity, to make easier its application onto a surface. A gel suspension can generally be readily screened onto a surface.
Where the polymeric support medium is wholly or substantially a solid, e.g. a more cross-linked carboxylic hydrophillic polymer or copolymer such as HEMA, the colloidal metal solution can be added directly thereto to form the suspension. The support medium may swell, but will return to size once the suspension is dried, e.g. in air or under vacuum, etc. Such support media could be provided in sheet form, which could be divided into a plurality of suitable matrices, (each one for subsequent repeatable analysis), once the colloidal metal solution is absorbed or suspended thereby.
Whilst any metal particles that could be used in Raman spectroscopy could be used in the colloidal metal solution of the present invention, such metal particles are generally either silver or gold.
According to a second aspect of the present invention, there is provided a solid matrix for use with surface-enhanced Raman spectroscopy, which matrix includes metal particles and polymeric support medium, preferably as a thin film.
Preferably, the solid matrix is a flat sheet or film or is located on a flat surface such as a plate, or on a single or multiple well plate.
The present invention clearly provides the ability to prepare a number of identical and stable SER(R)S active support surfaces, which can be stored over a relatively long term, and which can provide an active medium for use at any time, and whose variation should be sufficiently minimal as to wholly or substantially provide similar Raman spectra from the same analyte. Reproducibility of spectra is therefore now possible over much longer time periods than before, and a lot less time and effort should be involved in having to try and replicate the analyte support conditions each time a Raman spectrum is desired.
It is considered that the present invention could allow the use of Raman spectroscopy to be used much more quickly and more widespread than before, eg outside forensic laboratories, and much closer to the source of analytes. Its"" local use in forensics and/or medical is diagnostics is clearly possible.